This invention is related to pulse rate measurement systems, and particularly to an improved pulse rate measuring system that includes input coupled pole zero cancellation to rapidly and accurately develop pulse rate readings.
In a patent application, Ser. No. 965,816, entitled Heart Rate Measurement System, invented by Lanny L. Lewyn and filed Dec. 5, 1978, the same inventor on this application, a pulsed IR plethysmograph was disclosed and claimed having a photodiode and a gated circuit connected thereto for pulse-by-pulse cancellation of the asynchronous ambient light components of the photocurrent signal. A switched integrate and hold circuit was provided to respond to the current signal after ambient light cancellation to provide a signal representive of the blood pressure modulation. A switching transconductance element, an integrating operational amplifier and the integrate and hold circuit were included in a perfect second-order feedback loop to develop current pulses for cancellation of synchronous IR carrier steady state photocurrent resulting from the pulsed light source. Because of the size restraints on certain elements in the system and the objective of keeping individual stage DC gains low, especially in a watch, the differentiation in the perfect second-order loop necessarily had a relatively large time constant. The result of this long time constant differentiation was an undesirable shaping effect on the heart pressure wave at the output of the loop so that the amount of pulse rate uncertainy was greater than might have been desired. An arrangement as shown and described in the above-referenced patent application that would remove the effect of this long time constant third differentiation and result in a system with only double differentiation with a short time constant would be a substantial advance to the art.
Pole zero cancellation is generally known in network theory, at least as to remaining poles at the limits, as discussed in the book by E. A. Guillermin, Synthesis of Passive Networks, John Wiley and Sons, Inc., New York, 1957. Also, in the nuclear field, pole zero cancellation has been applied to some of the problems of pulse shaping in nuclear pulse amplifiers, as discussed in a paper by C. H. Nowlen and J. L. Blankenship published starting with page 1830 in the Review of Scientific Instruments, 1965 edition, Vol. 36, No. 12. This paper shows a system operating with a charge sensitive preamplifier that has overload conditions resulting from high energy particles that deposit a large amount of energy on the detector causing the charge sensitive preamplifier to go into an overload condition. The pole zero cancellation in in the system of the paper is to cancel the effect of the slow recovery of the charge sensitive preamplifier so that information can be reliably extracted from a single pulse. The objective of applicant's compensation by pole zero cancellation is different than taught in the prior art in that Applicant's system operates with DC coupling to a perfect second-order loop which has the purposes of suppressing the individual pulse of the pulse carrier and extracting only the information from variations in pulse-to-pulse amplitude. Thus, Applicant's compensation operates in a time frame that is slow compared to the pulse-to-pulse time interval. In contrast to Applicant's system, the nuclear pulse amplifier system of the paper uses pole zero cancellation to discard the remnants of a pressure pulse so that the system is totally cleaned when the next pulse is received. Further, Applicant's invention is to prevent excess noise and an undesirable shaping effect in the heart blood pressure wave which would lead to improper timing information while the pole zero cancellation in the paper is used primarily for measuring pulse amplitude rather than time. Accordingly, Applicant's type of compensating circuit including its function, is direct coupling to the signal source and its arrangement in conjunction with a perfect second-order loop input is not taught in the prior art.